fix(autocmd): skip empty comma-separated patterns properly

Problem: empty comma-separated patterns in an aupat aren't skipped correctly.
Solution: skip consecutive commas in an aupat.

Also simplify the logic.

src/nvim/api/autocmd.c

@@ -823,12 +823,10 @@ static Array get_patterns_from_pattern_or_buf(Object pattern, bool has_buffer, B
   if (pattern.type != kObjectTypeNil) {
     if (pattern.type == kObjectTypeString) {
       const char *pat = pattern.data.string.data;
-      size_t patlen = aucmd_pattern_length(pat);
+      size_t patlen = aucmd_span_pattern(pat, &pat);
       while (patlen) {
         kvi_push(patterns, CBUF_TO_ARENA_OBJ(arena, pat, patlen));
-
-        pat = aucmd_next_pattern(pat, patlen);
-        patlen = aucmd_pattern_length(pat);
+        patlen = aucmd_span_pattern(pat + patlen, &pat);
       }
     } else if (pattern.type == kObjectTypeArray) {
       if (!check_string_array(pattern.data.array, "pattern", true, err)) {
@@ -838,12 +836,10 @@ static Array get_patterns_from_pattern_or_buf(Object pattern, bool has_buffer, B
       Array array = pattern.data.array;
       FOREACH_ITEM(array, entry, {
         const char *pat = entry.data.string.data;
-        size_t patlen = aucmd_pattern_length(pat);
+        size_t patlen = aucmd_span_pattern(pat, &pat);
         while (patlen) {
           kvi_push(patterns, CBUF_TO_ARENA_OBJ(arena, pat, patlen));
-
-          pat = aucmd_next_pattern(pat, patlen);
-          patlen = aucmd_pattern_length(pat);
+          patlen = aucmd_span_pattern(pat + patlen, &pat);
         }
       })
     } else {

src/nvim/autocmd.c

@@ -165,7 +165,7 @@ static void au_show_for_event(int group, event_T event, const char *pat)
   // Empty pattern shows all autocommands for this event
   int patlen = 0;
   if (*pat != NUL) {
-    patlen = (int)aucmd_pattern_length(pat);
+    patlen = (int)aucmd_span_pattern(pat, &pat);
     if (patlen == 0) {  // Don't show if it contains only commas
       return;
     }
@@ -288,8 +288,7 @@ static void au_show_for_event(int group, event_T event, const char *pat)
       }
     }
 
-    pat = aucmd_next_pattern(endpat, 0);
-    patlen = (int)aucmd_pattern_length(pat);
+    patlen = (int)aucmd_span_pattern(endpat, &pat);
   } while (patlen);
 }
 
@@ -920,7 +919,7 @@ int do_autocmd_event(event_T event, const char *pat, bool once, int nested, cons
   }
 
   // Loop through all the specified patterns.
-  int patlen = (int)aucmd_pattern_length(pat);
+  int patlen = (int)aucmd_span_pattern(pat, &pat);
   while (patlen) {
     const char *endpat = pat + patlen;
 
@@ -966,8 +965,7 @@ int do_autocmd_event(event_T event, const char *pat, bool once, int nested, cons
       autocmd_register(0, event, pat, patlen, group, once, nested, NULL, cmd, &handler_fn);
     }
 
-    pat = aucmd_next_pattern(endpat, 0);
-    patlen = (int)aucmd_pattern_length(pat);
+    patlen = (int)aucmd_span_pattern(endpat, &pat);
   }
 
   au_cleanup();  // may really delete removed patterns/commands now
@@ -1105,46 +1103,28 @@ int autocmd_register(int64_t id, event_T event, const char *pat, int patlen, int
   return OK;
 }
 
-size_t aucmd_pattern_length(const char *pat)
-  FUNC_ATTR_PURE
+size_t aucmd_span_pattern(const char *pat, const char **start)
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
 {
-  if (*pat == NUL) {
-    return 0;
+  // Skip leading commas.
+  while (*pat == ',') {
+    pat++;
   }
 
-  const char *endpat;
-
-  for (; *pat; pat = endpat + 1) {
-    // Find end of the pattern.
-    // Watch out for a comma in braces, like "*.\{obj,o\}".
-    endpat = pat;
-    // ignore single comma
-    if (*endpat == ',') {
-      continue;
+  // Find end of the pattern.
+  // Watch out for a comma in braces, like "*.\{obj,o\}".
+  const char *p = pat;
+  int brace_level = 0;
+  for (; *p && (*p != ',' || brace_level || (p > pat && p[-1] == '\\')); p++) {
+    if (*p == '{') {
+      brace_level++;
+    } else if (*p == '}') {
+      brace_level--;
     }
-    int brace_level = 0;
-    for (; *endpat && (*endpat != ',' || brace_level || endpat[-1] == '\\'); endpat++) {
-      if (*endpat == '{') {
-        brace_level++;
-      } else if (*endpat == '}') {
-        brace_level--;
-      }
-    }
-
-    return (size_t)(endpat - pat);
   }
 
-  return strlen(pat);
-}
-
-const char *aucmd_next_pattern(const char *pat, size_t patlen)
-  FUNC_ATTR_PURE
-{
-  pat = pat + patlen;
-  if (*pat == ',') {
-    pat = pat + 1;
-  }
-  return pat;
+  *start = pat;
+  return (size_t)(p - pat);
 }
 
 /// Implementation of ":doautocmd [group] event [fname]".

test/functional/autocmd/autocmd_spec.lua

@@ -800,4 +800,51 @@ describe('autocmd', function()
       fn.execute('autocmd BufEnter <buffer=1>,bar,bar,foo,foo,<buffer>,<buffer=6>,<buffer>')
     )
   end)
+
+  it('parses empty comma-delimited patterns correctly', function()
+    exec [[
+      autocmd User , "
+      autocmd User ,, "
+      autocmd User ,,according,to,,all,known,,,laws,, "
+    ]]
+    api.nvim_create_autocmd('User', { pattern = ',,of,,,aviation,,,,there,,', command = '' })
+    api.nvim_create_autocmd('User', {
+      pattern = { ',,,,is,,no', ',,way,,,', 'a,,bee{,should be, able to,},fly' },
+      command = '',
+    })
+    eq(
+      {
+        'according',
+        'to',
+        'all',
+        'known',
+        'laws',
+        'of',
+        'aviation',
+        'there',
+        'is',
+        'no',
+        'way',
+        'a',
+        'bee{,should be, able to,}',
+        'fly',
+      },
+      exec_lua(function()
+        return vim.tbl_map(function(v)
+          return v.pattern
+        end, vim.api.nvim_get_autocmds({ event = 'User' }))
+      end)
+    )
+    eq(
+      dedent([[
+
+      --- Autocommands ---
+      User
+          there
+          is
+          a
+          fly]]),
+      fn.execute('autocmd User ,,,there,is,,a,fly,,')
+    )
+  end)
 end)